I've said this many times and will still stand by it 100%. It has yet to fall me...
Get a multimeter, and check continuity from each key contact on the flex mylar circuit to the edge that plugs into the connector.
Anywhere there is a break/open you start to narrow it down, closer and closer until you pinpoint the exact open.
Then use some copper trace paint (I just get mine from automobile rear-window defroster repair kits, permetex works great!) to bridge the gap. Sometimes there are many! I use a toothpick to paint the trace.
Be patient and use the multimeter... pinpoint, paint it. And repeat. Until you have a full continuity reading on your multimeter for each mylar key contact.
Plug it in, and you are good to go.
I started using this method many years ago, intending it just to be temporary until I could get new mylar. But I've done this to at least 8 systems over many years, and NONE of them have ever failed again!
Eric